Bleaching of ground wood pulp with preliminary bisulphite treatment



Patented July 21, 1942 BLEACHING OF GROUND WOOD PULP WITH PRELIBIINARYBISULPHITE TREATLIENT Joseph S. Reichert, Samuel A. McNeight, and

Howard L. Potter, Niagara Falls, N. Y., assignors to E. I. du Pont deNemours & Company, Wilmington, Del., "a corporation of Delaware NoDrawing. Application February 16, 1940, Serial No. 319,238

7 Claims.

This application is a continuation-in-part of our patent applicationSerial No. 297,150, filed September 1939, now Patent No. 2,199,376,issued April 30, 1940.

The invention relates to the bleaching of paper pulp and, moreparticularly, to the bleaching of ground wood pulp. In presentcommercial practice, ground wood pulp, usually with other paper pulps,is employed in the manufacture of papers such as book and magazine paperand newsprint paper. Paper prepared from ground wood pulp, in additionto containing the relatively inexpensive ground wood pulp,generallypossesses improved printing properties and a better opacity ascompared with paper prepared solely from the so-called chemical pulps assulphite pulp. Improved printing properties, increased absorbency, andsmoother surface all result when ground wood pulp is utilized in themanufacture of paper.

Unfortunately, there is no method now known to the paper pulp industriesby which ground wood pulp of high brightness and uniform high qualitycan be prepared. In view of the inherent advantages of ground wood as araw material in the manufacture of paper, paper manufacturers have forsome time been interested in a commercially practical method forpreparing ground wood pulp of high brightness under conditions whereinthe wood fiber would not be deleteriously weakened or the opacity orprinting qualities of the resulting paper impaired by the treatment. Theprincipal object of this invention is, therefore, the development of apractical commercial method for the preparation of ground wood pulpofhigh brightness, which pulp is suitable, either alone or in admixturewith other pulps, for use in the preparation of paper of satisfactorylight color and increased whiteness retentivity.

While ground wood pulp has, to some extent, been subjected to singlestage bleaching proc-- esses in which a reducing agent is employed,these attempts to secure satisfactory bleaching have been generallyunsuccessful. -Any increased brightness in the ground wood pulpresulting from the usual acid sulphite bleaching operation, whereinreducing agents such as hydrosulphites and bisulphites are employed, hasgenerally not been carried over into the paper manufactured therefrom.At best, only a temporary whiteness has been imparted to the paper.Moreover, ground wood pulp bleached by the acid sulphite method, thesingle stage bleaching process uticloses a new and improved method forthe' bleaching of ground wood pulp which yields an improved wood pulp ofhigh quality and satisfactory brightness. The single stage bleachingprocess disclosed in that application is, forthese reasons, a distinctimprovement over the single stage bleaching methods now known to theart. We have found, however, that even better bleaching results areobtained and a ground wood pulp of high brightness and improvedproperties is secured, when there is employed a two-stage bleachingprocess, the first stage being the usual acid sulphite reducingbleaching treatment now known to the art and the second stage involvingthe use of our improved oxidizing bleach. Our improved process is,therefore, directed in its broader aspects to a two-stage bleachingoperation which includes, as the first step, treatment of the groundwood slurry with the usual acid sulphite bleaching liquor, i. e.,sulphur dioxide in acid solution, followed by a second step wherein thepulp is treated with an alkaline solution containing a peroxide. Theacid sulphite treatment of the first stage may, of course, be carriedout by treating the ground wood pulp slurry with a bisulphite or ahydrosulphite in a solution rendered acid, if necessary, by the additionof a mineral acid such as sulphuric acid, or of an acidic substance suchas alum, these agents forming sulphurous acid in the solution. Thesecond stage, or oxidizing bleaching step, employing an alkalinesolution of hydrogen peroxide or its equivalent, is substantiallyidentical with the single stage bleaching process described in ourco-pending patent application and is carried out under the conditionsdescribed in that patent application.

In order that our two-stage process for the bleaching of ground woodpulp may be entirely clear, we will describe the individual stepsthereof in some detail. The first step. involves treatment of the groundwood pulp in acid solution with sulphur dioxide or some agent yieldingsulphur dioxide under the conditions of treatment. In accordance withthe usual practice in the paper and paper pulp industries, this ishereinafter reierred to as the acid sulphite treatment.

Ordinarily, any reducing bleaching treatment employing sulphur dioxideor its equivalent now lizing an agent yielding sulphur dioxide in acidknown to h art may beutilized as the first step in our two-stage processfor the bleaching of ground wood pulp. Thus, the bleaching agent may bea bisulphite such as sodium bisulphite, or a hydrosulphite such assodium hydrosulphite, employed in acid solution. The acid employed maybe a mineral acid such as sulphuric acid or an acidic substance such asalum, or other suitable acidic material. The pH of the solution duringthe acid sulphite bleach should be between 3 and 6, the suspension ofground wood in the bleached liquor being acid in reaction during theentire step. Generally, it is to be preferred that the pH fall somewherewithin the range 4.0 to 5.5, and we regard a pH of 5.0 or close theretoas the optimum pH during this step.

As is now well-known in the wood pulp art, the amount of the acidsulphite bleaching agent employed is not critical since the bleachingaction is of the instantaneous or flash type, at most requiring but ashort period of contact, If desired, however, the pulp may be allowed toremain with the acid sulphite bleaching agent for a period of severalhours. It is frequently desirable to allow the ground wood pulp toremain in contact with the acid sulphite bleaching liquor for a periodof time suflicient to result in the decomposition and elimination of themajor portion of the acid bleaching solution. This is especiallyadvantageous when washing of the ground wood pulp at the conclusion ofthe acid sulphite bleach is not contemplated. Sulphur dioxide and aperoxide such as hydrogen peroxide or alkali metal peroxide will reactwhen brought together, Under these circumstances, if a large amount ofresidual bisulphite or hydrosulphite remains in the wood pulp, a certainquantity of the hydrogen peroxide or equivalent peroxide is destroyedwithout effecting any substantial bleaching by reaction with the sulphurdioxide, if the pulp is directly treated in the second stage withoutintermediate washing. While intermediate washing between the two stagesof our process is desirable, we do not ordinarily employ such washingfor the reason that it introduces an additional step and furtherexpense. We therefore ordinarily operate so that but a small excess ofthe bisulphite or hydrosulphite, or none at all, remains in the pulp atthe time that pulp is contacted with the solution containing theperoxide, so that the loss of peroxide by reaction is relatively smallin extent and well within permissible limits.

The second stage of our process, involving treatment of the ground woodwith an alkaline peroxide, will now be described in some detail, thisstep constituting substantially the same procedure described inconjunction with the single stage bleaching process of our pendingapplication, Serial No. 297,150, now Patent No. 2,199,376.

The active agent in the second stage is a peroxide in alkaline aqueoussolution, this solution ordinarily also containing a stabilizing andbuffering agent such as sodium silicate. An alkaline solution ofhydrogen peroxide can be employed with entire success. However, weprefer to use an alkali metal peroxide such as sodium peroxide. Asolution of an alkali metal peroxide has been found to be equivalent inaction in our process to that of an alkaline solution of hydrogenperoxide, and since sodium peroxide on an active oxygen basis isrelatively less expensive than hydrogen peroxide, we prefer to use thealkali metal peroxide. In place of sodium peroxide or hydrogen peroxide,9. solution ofa peroxygen compound such as sodium perborate may also beutilized. As all these agents in alkalin solution are equivalent inaction in our process, the following description of the preferredprocedure employed in the second stage of our improved bleaching methodwill refer specifically to sodium peroxide as the bleaching agent. Whenother oxidizing bleaching agents such as hydrogen peroxide or alkalimetal perborates are used, the amount of those bleaching agentsnecessary is readily determinable from the amounts herein specified forsodium peroxide. The use of a bleaching agent other than sodium peroxidewill involve adjustment of the pH of the bleaching bath within thelimits hereinafter specified, as well as the utilization of suchquantities of the respective active oxygen-yielding material as arenecessary to yield an active oxygen concentration substantiallyequivalent to the active oxygen concentration resulting from the use inour process of the specified amounts of sodium peroxide.

Our extensive experiments in the bleaching of ground wood pulp have beencarried out principally with commercial aqueous suspensions of groundwood which are of from 3 to 7% pulp concentrations. In the descriptionof the second stage of our process which follows, we have in mind pulpsof approximately those consistencies. Of course, during the first stageof our process, it is not necessary to maintain any specifiedconcentration of pulp in the aqueous bleaching solution, the acidsulphite treatment being carried out under conditions now well known inthe single stage acid sulphite bleaching process as now known to thepaper and pulp industries. During the second stage of our process, whilewe prefer to employ a pulp consistency of from 3 to 7%, there is, ofcourse, nothing inherent in our process which restricts it to treatmentof ground wood slurries falling within the specified consistencies. Itmay be noted that wood pulps of from 3 to 7% consistency are generallyprevalent in the paper pulp and paper industries.

When utilizing sodium peroxide as the bleaching agent in the secondstage of our process we have found that it is desirable to utilize anamount of sodium peroxide not over approximately 7.5% of the weightofthe dry ground wood being treated. In specifying the amounts of chemicalagents utilized, the ordinary practice of the paper pulp industry isfollowed, the amounts being expressed as a percentage based on theweight of the dry ground wood pulp being treated. For example, bleachingwith 5% sodium peroxide means that there will be present in the bleachbath five pounds of sodium peroxide for every one hundred pounds ofground wood being treated. Similarly, bleaching with 2% sodium peroxidemeans that the bleach liquor contains two pounds of sodium peroxide perone hundred pounds of dry ground wood present in the slurry beingtreated. This method of expressionis customary in the paper pulpindustries, and will be followed here.

While ordinarily we prefer to use about 2 or 3% sodium peroxide or twoto three pounds of sodium peroxide per one hundred pounds of the dryweight of the ground wood present in the 'pulp being treated, it ispossible to utilize higher percentages of sodium peroxide to givegenerally a greater degree of whiteness in our two step process.Similarly, lower percentages of sodium peroxide may be employed, butresult, generally, in a less increase in brightness. For securingimprovements in brightness satisfactory for commercial operations, wehave found it inadvisable to utilize less than 0.25% sodium peroxide(onequarter pound of sodium peroxide per one hundred pounds of groundwood present in the pulp being treated). Amounts of 0.50 pound of sodiumperoxide per one hundred pounds of ground wood (0.50%) have been foundin many instances to yield a satisfactory bleached pulp when employed asthe second bleaching agent in our process where the pulp has first beentreated with the acid sulphite bleach. While the higher concentrationsof sodium peroxide, concentrations up to 7.5% sodium peroxide based onthe dry weight of the pulp or even higher, as for example 10% sodiumperoxide, when utilized in the second step of our two-stage process willgenerally result in greater whiteness and to some extent a higher degreeof whiteness retentivity, we have generally found it inadvisable toexceed the figure of 7.5% sodium peroxide concentration, i. e., 7.5pounds of sodium peroxide per One hundred pounds of dry ground wood.Percentages in excess of 10.0% will practically always result in butslight increase in whiteness when utilized in our two-stage process, andwill tend to increase the cost of the bleaching-treatment to one whichwill no longer render the process attractive from the been utilized andtemperatures as high as 120 F.

are also practical. The temperature range that We have found desirable,80 to 120 F., does not differ materially from ordinary room temperaturesand the use of such relatively low temperatures in bleaching the groundwood pulp constitutes a distinct departure in the art. In view of theinherent difiiculty of securing a relatively permanent bleach, we havefound that the maintenance of a temperature preferably in theneighborhood of 100 F., and in an event within the range 80 to 120 F.,is essential.

Another essential factor which must be strictly regulated in order tosecure the desired degree of bleaching during our second step, and thedesired whiteness retentivity, is the pH of the mixture of ground woodpulp suspension and bleaching agent. The regulation of this pH duringthe major portion of the oxidizing bleaching operation within a verynarrow range is essential if a satisfactory product is to be secured. Wehave found that the pH must fall strictly within the range 10.0 to 11.0during the major portion of the bleaching if results of commercialacceptability are to be obtained. Generally speaking, at the lowertemperatures, such as temperatures of 80 F. or thereabouts, relativelyhigher pH values such as a DH value up to 11.0 or even somewhat highercan be maintained without the danger of excessive injury to the pulp.

Ordinarily we prefer to maintain the pH value within the range 10.5 to10.8, although at temperatures as low as 80 F., as previously stated,the pH may safely be as high as 11.0 or even higher without interferingwith the desired bleaching. If the pH value is allowed to go too high,not only is the bleaching not satisfactory, but there is the possibilityof a distinct yellow color being formed in the pulp, which yellow coloris very difficult to bleach out or remove. We have observed, on theother hand, that pH values below 9.5 will generally result ininconsequential and unsatisfactory bleaching and will not produce a pulpof the desired brightness. For most purposes, therefore, it is essentialto maintain a PH of not below about 10.0 in order to secure i andbleaching agent during the second step of our process is very desirableif not essential as it may provide part of the alkalining agent, andalso acts to some extent as abuflering material to maintain the pH valuewithin the desired range 10.0 to 11.0 during the major portion of thebleaching. Surprisingly enough, we have found that the amount of sodiumsilicate which should be present for satisfactory bleaching dependslargely on the concentration of sodium peroxide or equivalent agentutilized. Thus, in employing 1 or 2% sodium peroxide, i. e., 1 to 2 lbs.of sodium peroxide per 100 pounds dry weight of the ground wood pulp, itis desirable to have present at least 6% of sodium silicate. (The sodiumsilicate concentration is specified in the same manner as the sodiumperoxide concentration, and a 6% concentration means 6 pounds of sodiumsilicate per 100 pounds dry weight of the ground wood being treated.) Ifamounts of sodium silicate less than 6% are present when the sodiumperoxide concentration is from 1 to 2% NazOz, the degree of brightnessof the pulp at the conclusion of the process has been found to besomewhat less than that obtainable by the use in the second stage of ourbleaching process of the proper amount of sodium silicate. Similarly,when the sodiumperoxide concentration utilized in the second step is inexcess of 2%, we

have found that the minimum quantity of sodium silicate which should bepresent if maximum brightness is to be secured should be about 10%. Forexample, when treating ground wood pulp at 100 F. durin the second stageof our process with a bleach liquor containing 3% sodium peroxide, thereshould also be present in that bleach l quor about 10 pounds of sodiumsilicate per 100 pounds dry weight of the ground wood. It may be statedthat the values given for the sodium' silicate content, if maximumimprovement in brightness is to ,be secured during the second stage, arebased on certain standard commercial sodium silicates, such as 42 B.Grasselli No. 30.

It is, of course, understood that the improved process is in no senserestricted to the use of this silicate, or to sodium silicate of anyspecified gravity or manufacture.

The sodium silicate concentration in the bleaching liquor willordinarily not exceed 10% even for the higher concentrations ofperoxide, i. e., concentrations up to 10.0% NazOz. Even for very lowconcentrations of peroxide, concentrations as low for example, as 0.50%or 0.025%, it has been found desirable for most satisfactory bleachingand improvement in brightness during the second stage of our process tohave substantially 6% sodium silicate present. The sodium silicate rangemay, therefore, be said to extend from 6% sodium silicate to' 0% sodiumsilicate, the particular amount selected being determined by theconcentration of sodium peroxide or m ivalent bleaching agent utilizedin our process.

The presence of sodium silicate is also desilable as it furnishesprotection for the metallic equipment against corrosion by alkalinecompounds present in the bleach liquor, when that liquor comes intocontact with metals. Moreover, it serves the important and primaryfunction of assisting in maintaining the pH within the range given, tosome extent'serving as both an alkalining and buffering agent, and as astabilizing agent.

We have found that a period of contact betwee the ground wood pulp andthe oxidizing bleaching solution of from 2 to 4 hours or even longer isdesirable in order that satisfactory brightening may be secured. Aspreviously specified, the pH should fall during the greater portion ofthe bleaching at some value within the range of 10.0 to 11.0. However,it is not detrimental to the quality of the ground wood pulp if the pHdrops during the latter portion of the step as low as 9.5, and in someinstances even as low as 9.0,.as the major portion of the bleaching willhave been substantially completed by this time. The major portion of thebleaching may, for example, be complete within a very few minutes, eventhough in order to secure a more eiiective utilization of the peroxide,it may be desirable to allow the wood pulp to remain in contact with thebleach liquor for periods of four hours or even longer. As long as thepH is maintained within the range of 10.0 to 11.0 during the majorportion of the bleaching, it is immaterial if the pH value falls below10.0 even for a considerable period of time, as long as the majorportion of the bleaching has been completed before the decrease in thepH value of the suspension occurs.

In order to secure satisfactory contact between the ground woodsuspension and the alkaline oxidizing agent during the second step ofour process, it is desirable to subject the suspension to initialstirring to an extent sufiicient to insure thorough mixing; Continuousstirring is not essential, but agitation in some manner sufilcient tosecure thorough mixing at the start of the bleaching process may beregarded as a desirable element in our process. In place of stirring-bymeans of a mechanical stirrer, contact can, of course, be secured byother means well known in the industry such as the discharge of gasbubbles in the suspension, shaking the container, etc. A At theconclusion of the second step or our improved bleaching process, thepulp is immediately ready, without the necessity for further treatment,for use in the manufacture of paper or for other utilization in thepaper industry. It may sometimes be desirable to subject the ground woodpulp at the conclusion of the process to washing, with water or withaqueous liquids, but washing is not an essential step in our process andneed not be carried out at any phase thereof. Generally, the pH of themixture which is converted into paper, called the "furnish" in thepaper-making industry, is adjusted by the addition of acid or acidsubstances as part of the process of paper manufacture.

By the use of our two stage bleaching process, it is possible to securean improvement in brightness not attainable by the use of the singlestage bleaching process described in our previously rei erred toco-pending patent application. Our improved bleaching method possessesthe added advantage that it renders permanent at least part of the colorimprovement securable by use of the acid sulphite treatment, a resultthat has not previously been attained when ground wood pulp has beensubjected to the bleaching action of reducing bleaching agents.Moreover, the impermanent character of the improvement in colorcharacteristic of the acid sulphite bleach is not evident in pulpbleached by the two stage method, since the final step in our process,involving the use of the oxidizing alkaline bleaching agent, serves tofix and render permanent at least part of the improvement in brightnesssecured during the acid sulphite treatment. Since not only a high degreeof'brightness, but also a high whiteness retentlvity is thus secured,ground wood pulp bleached in accordance with our two-stage process is ofquality much superior to any that can be attained by the use ofbleaching agenm of equivalent cost in any single stage method, includingthe single stage oxidizing bleaching method of or co-pendingapplication. The degree of brightness attained is one that, of course,has never been previously secured with any acid sulphite bleachingtreatment. To those skilled in the art who are familiar only with thetransitory and impermanent brightness secured with sulphur dioxide inacid. solution, the whiteness retentivity of the bleached ground woodpulphas been universally regarded as remarkable.

As an example of our improved two-stage process for the bleaching ofground wood pulp, the following may be given:

Example Ground wood, having a brightness as measured by the GeneralElectric reflectance meter of 59.5 was treated in accordance with thefollowing twostage process.

The ground wood pulp suspension of 5% consistency was treated withapproximately 0.5% sodium bisulphite in acid solution, in accordancewith the usual acid sulphite bleach utilized in the paper pulp industry.The pH of the suspension during this step was 4.0. The suspension wasallowed to stand for approximately one hour in order that the greaterportion of the bisulphite might be used up during this step.

The ground wood pulp was now treated with 3% of sodium peroxide, i. e. 3pounds of N820: per pounds of ground wood pulp, based on the dry weightof the ground wood. The pulp consistency was maintained at approximately5%, and the pH was adjusted to 10.5 by the addition of sulphuric acidand of sodium silicate. The amount of sodium silicate added wasapproximately 10%, based on dry weight of pulp. The temperature wasmaintained at 100 F. and the second step bleaching operation wascontinued until substantially all of the sodium peroxide had been usedup.

The ground wood pulp, at the conclusion of the two-stage process, had abrightness of 69.5 as measured by the General Electric reflectancemeter. The pulp was suitable for immediate use in th manufacture ofpaper.

As a comparison, the second step of the process was repeated with asample of ground wood pulp which had not been previously subjected tothe first stage treatment with the acid sulphite bleach liquor. Allother conditions maintained were identical with those specified. Thebrightness of this ground wood pulp at the conclusion of the singlestage oxidizing bleach was found to be 67.0 as measured by the GeneralElectric reflectance meter. It is evident that by the use of the acidsulphite first stage bleach, the brightness of the pulp was increased2.5 points.

To those skilled in the art, many modifications and widely differentembodiments and applications of our invention in the general field ofbleaching ground wood pulp will be readily apparent. It should beunderstood that the details of our preferred process as described,including the concentrations, conditions under which the bleaching iscarried out and other factors involved, may be subjected to some changewithout departing from either the spirit or scope of our invention. Itis intended that our invention is not to be restricted to specificpreferred embodiments, minor details, or specific modes of operation,other than as necessitated by the prior art and appended claims.

We claim:

1. The process of bleaching ground wood pulp in order to prepare aground wood pulp of high brightness for use in the paper making artwhich comprises: subjecting said ground wood pulp to an acid sulphitebleach; and thereafter subjecting said ground wood to the action of anaqueous alkaline oxidizing bleach liquor containing a peroxide, saidaqueous alkaline liquor having a pH within the range of 10.0 to 11.0during the major portion of said oxidizing bleach step, the temperatureduring said latter oxidizing bleach step being maintained within therange, 80 to 120 F.

2. The process of preparing a ground wood pulp of high brightness foruse in the paper making art which comprises: subjecting said ground woodtothe actionof an acid sulphite bleach; and thereafter subjecting saidground wood to the action of an oxidizing bleach liquor containingsodium peroxide, said oxidizing bleach liquor having a pH falling withinthe range 10.0 to 11.0 during the major portion of said oxidizingbleaching step, the mixture of ground wood and oxidizing bleach liquorbeing maintained at a temperature within the range 80 to 120 F. duringthe period wherein the major portion of the bleaching during saidoxidizing bleach step occurs.

3. The process of bleaching ground wood pulp in order to secure aproduct of high brightness suitable for use in the paper making industrywhich comprises: subjecting said ground wood to the action of an acidsulphite bleach bath; and thereafter treating said pulp in an aqueoussuspension of from 3% to 7% pulp concentration with an alkalineozidizing bleach liquor containing sodium peroxide and sodium silicate,said sodium peroxide being present in amounts up to 10 parts by weightper 100 parts dry weight of said ground wood, the temperature of saidaqueous suspension of ground wood and alkaline bleach liquor falling,during the major portion of said oxidizing bleaching step, within therange 80 to 120 F., and the pH of said suspension, during that portionof said oxidizing bleaching step wherein the major portion of saidoxidizing bleach occurs, falling within the range 10.0 to 11.0.

4. The process of bleaching ground wood pulp in order to prepare aground wood pulp of high brightness for use in the paper making artwhich comprises: subjecting said ground wood pulp to an acid sulphitebleach; and thereafter subjecting said ground wood to the action of anaqueous alkaline oxidizing bleach bath containing a peroxygen compoundselected from the group which consists of hydrogen peroxide, the alkalimetal peroxides, and the alkali metalperborates, said aqueous alkalineliquor having a pH within the range 10.0 to 11.0 during the majorportion of said oxidizing bleach step, the temperature during saidlatter oxidizing bleach step being main tained within the range to F.

5. The process of bleaching ground wood pulp in order to prepare aground wood pulp of high brightness for use in the paper making artwhich comprises: subjecting said ground wood pulp to an acid sulphitebleach; and thereafter subjecting said ground wood to the [action of anaqueous alkaline oxidizing bleach liquor containing a peroxide andsodium silicate, said aqueous alkaline liquor having a pH within therange 10.0 to 11.0 during the major portion of said oxidizing bleachstep, the temperature during said latter oxidizing bleach step beingmaintained within the range 80 to 120 F.

6. The process of preparing a ground wood pulp of high brightness foruse in the paper making art which comprises: subjecting said ground woodto the action of an acid sulphite bleach; and thereafter subjecting saidground wood to the action of an oxidizing bleach liquor containing analkali metal peroxide, said oxidizing bleach liquor having a pH fallingwithin the range 10.0 to 11.0 during the major portion of said oxidizingbleach step, the mixture of ground wood and oxidizing bleach liquorbeing maintained at a temperature within the range 80 to 120 F. duringthe period wherein the major portion of the bleaching during saidoxidizing bleach step occurs.

7. The process of preparing a ground wood pulp of high brightness foruse in the paper making art which comprises: subjecting said ground woodto the actionof an acid sulphite bleach; and thereafter subjecting saidground wood to the action of an oxidizing bleach liquor containinghydrogen peroxide, said oxidizing bleach liquor having a pH fallingwithin the range 10.0 to 11.0 during the major portion of said oxidizingbleaching step, the mixture of ground wood and oxidizing bleach liquorbeing maintained at a temperature within the range 80 to 120 F. duringthe period wherein the major portion of the bleaching during saidoxidizing bleach step occurs.

